Printed circuit boards are commonly fabricated by solder bonding each lead of an electronic component to a corresponding one of a plurality of selectively interconnected, metallized areas on a major surface of an insulative substrate, such as FR-4 or the like. To assure that each circuit board operates properly, each board is routinely tested following soldering of the component leads. Typically, the testing is carried out with the aid of an automatic testing machine which serves to launch pulsed test signals into the circuit board via a transmission line, such as a coaxial cable. Once the test signals are launched, the testing machine monitors the circuit board to sense the phase and amplitude of each signal returned from the board in response to each test signal. The phase of each response signal is measured in accordance with the amount of time the signal lags the corresponding test signal. By monitoring the phase and amplitude of the response signals, the testing machine can determine if the circuit board is operating correctly.
Presently, there is a trend towards higher component operating frequencies which allow for higher circuit board operating frequencies. As the operating frequency of the circuit board increases, so too must the frequency of the test signals launched into the board to assure testing of the board at its operating frequency. At very high frequencies, the propagation delay of signals traveling between the testing machine and the circuit board under test often is a significant source of error during testing. Therefore, it is desirable to compensate the testing machine for such propagation delays.
To accurately compensate for the propagation delays, the magnitude of such delays must be known. In the past, such propagation delays have been measured by the technique of time domain reflectometry which relies on the principle that a signal launched into one end of a circuit path is reflected back from the other end of the path when left open or unconnected. However, prior art calibration techniques are generally very complex and do not afford a high degree of precision, which is necessary to measure propagation delays at very high frequencies.
Therefore, there is a need for a technique for automatically measuring the propagation delays incurred by signals traveling along a circuit path with a high degree of precision using time domain reflectometry techniques.